The present invention relates to tissue-stimulating leads for use with a tissue-stimulating device, such as a cardiac pacemaker. More particularly, the present invention relates to a endocardial lead having a sealed, low-friction extendable/retractable screw-in electrode tip.
The advantages of screw-in cardiac leads are well known in the art. Such leads provide "active fixation" of the electrode tip to the tissue that is to be stimulated. This is in contrast to passive fixation devices, wherein the electrode tip may not be securely held against the body tissue until tissue ingrowth can occur. Epicardial leads employing screw-in tips have long been used in the art. In recent years, screw-in tips have also been used to secure endocardial leads. However, because the screw-in tip of such endocardial leads typically protrudes from a distal end of the lead, extreme care must be exercised when inserting the lead through a vein in order to prevent serious damage to vascular structure. Typically, such leads must be turned or rotated in a prescribed direction at the same time that they are being inserted into a vein in order to prevent the tip from engaging vascular structure at an undesired location.
Because of the problems and inherent dangers associated with a protruding screw-in tip of an endocardial lead during lead implantation, there have been developed in recent years endocardial leads having extendable/retractable screw-in electrode tips. A simple construction of such a lead incorporates a sliding carrier that is inserted into a tip housing at the distal end of the lead. A screw-in tip is attached to the distal end of the carrier. Prior to placement of the lead, the carrier is axially pushed, from the distal end of the lead, to a retracted position such that the screw-in tip does not protrude from the distal end. Once the lead has been positioned for placement, the screw-in tip is extended by axially pushing the carrier, from the proximal end of the lead through the use of a stylet, to an extended position so that the screw-in tip does protrude from the distal end.
Another type of extendable/retractable lead is the Medtronic Model 6957 lead. This lead incorporates a terminal pin/conductor-coil/electrode that rotates within polyether urethane insulation tubing. The screw-in tip is retracted within a housing at the distal end of the lead and remains in its retracted position until the distal tip is positioned at its desired tissue-stimulating location. Once at this location, the tip is extended by rotating the terminal pin approximately 8 to 10 turns, which rotation translates to approximately one turn at the distal or screw-in tip end of the lead, thereby effectuating the desired active fixation by screwing in the protruding screw-in tip.
It is also known in the art to use a lead similar to the Model 6957 described above, except that once positioned in its desired tissue-stimulating location, which positioning is typically realized through the aid of a conventional stylet, the conventional stylet is removed and a special stylet is inserted. This special stylet has a screw-driver like blade that engages a screw mechanism housed in the tubular housing at the distal end of the lead. To effectuate the desired extension of the screw-in tip of the lead, the lead is held steady while the special stylet is turned, thereby screwing the electrode-tip into the tissue at the desired location. In this particular embodiment, the fixation screw is not part of the electrode, that is, it is not electrically connected to the electrode tip. Rather, the screw-in tip serves merely as a mechanism for holding the distal tip, which includes a ring electrode, against the tissue.
Despite the advantages of the screw-in lead, numrous problems remain with its use. A relative large amount of external force is required in order to move the screw-in tip to its extended position. This is especially true for the simpler type constructions that utilize a sliding carrier to which the active fixation means is attached. These large forces are needed, of course, in order to overcome the friction that exists between the sliding carrier and the distal tip housing. Moreover, once the tissue has been penetrated by the screw-in tip, it is not uncommon for blood to leak from the fixation area into the lumen of the lead. The leaking of such blood into the lumen of the tissue-stimulating lead is undesirable. Further, especially when the tissue-stimulating lead is a cardiac lead that is positioned in heart tissue, the motion of the heart tissue (as the heart beats) can cause some loosening of the lead. Such loosening is not major, especially relative to passive fixation leads, but any amount of loosening can eventually allow scar tissue to build up between the electrode tip and the tissue. This scar tissue disadvantageously represents an increase in the resistance of the tissue-stimulating system, thereby necessitating an increase in the power level of the stimulating pulse in order to effectuate the desired stimulation. While modern tissue-stimulating devices can readily accomodate an increase in the stimulating pulse power level, such increases are disfavored because they represent an added drain on the batteries of such devices, thereby decreasing their useful life.
In view of the above, it is evident that there is a need in the art for an active fixation tissue-stimulating lead that avoids the problems of applying large forces in order to move the screw-in tip to its extended position, leaking of blood into the lumen of the lead during installation, and the loosening of the lead subsequent to installation. The present invention addresses these and other problems.